1. Field Of The Invention
The invention relates to data projection systems, and more particularly to providing an efficient and uniform source of polarized light in liquid crystal-based projectors.
2. Description of the Related Art
Data projectors, also known as digital projectors, are used for projecting images, usually color images, onto a screen. These projectors generally employ one of two types of data projection technologies. First, there are projectors using liquid crystal display (LCD) panels that act as the image forming device. LCD panels can be grouped into two categories: transmissive panels or reflective panels. Transmissive panels are constructed with front and back glass substrates so that light can travel through the panel. Reflective LCD panels are constructed with a front glass substrate and a reflective silicon back substrate. Projectors using transmissive LCD panels are generally referred to as LCD projectors. Projectors using reflective LCD panels are sometimes referred to as LCOS projectors. (LCOS stands for Liquid Crystal on Silicon). Second, there are projectors using reflective digital light processing (DLP) chips. These silicon based chips have movable micro-mirrors constructed on them that act as the image forming device. These projectors are generally referred to as DLP projectors.
Transmissive and reflective liquid crystal display (LCD) projectors typically include a light source emitting a randomly polarized white light. The light is transmitted through an LCD panel which modulates the light according to the desired image data. The resultant light is projected onto a screen for display. For color applications, a single panel or a three-panel LCD projector can be used. In a three-panel LCD projector, incident white light is typically separated into three primary color light components (such as red, green and blue) using prisms or color dichroic mirrors. Each color component passes through an associated LCD panel and is thus separately modulated. The modulated color components are recombined using a color combination prism and projected to produce a color image. In a single panel LCD projector, a color LCD panel can be used. Alternately, conventional methods for modulating different color light components onto the LCD panel, such as by using a color wheel, can be used.
FIG. 1 is a schematic diagram of a conventional single panel LCD projector. LCD projector 10 includes a parabolic lamp 12 as the light source emitting randomly polarized white light. A bank of lens arrays 14 collects the white light generated by lamp 12 and directs the light onto a polarizing converting system (PCS) plate 16. PCS plate 16 includes a column of polarizing beam splitter (PBS) elements for converting randomly polarized light from lamp 12 to a single polarization. In operation, the incident white light is split into s-polarized light and p-polarized light at the 45xc2x0 reflecting surface of the polarizing beam splitter element. The p-polarized light is transmitted and s-polarized light is reflected at this surface. The p-polarized light is then transmitted through a half-wave retarder which converts the p-polarized light to s-polarized light. This first beam, now s-polarized, exits PCS plate 16. Meanwhile, the original s-polarized light, separated from the p-polarized light, is reflected a second time in the PCS elements and exits PCS plate 16 as s-polarized light at a distance W from the first beam of s-polarized light. In this manner, PCS plate 16 converts the incoming white light into s-polarized light which is directed to a condenser lens 18. PCS plate 16 using PBS elements can achieve polarization at about 80% efficiency.
Condenser lens 18 focuses the s-polarized light onto a transmissive LCD panel 20. LCD panel 20 typically includes an input polarizer, a liquid crystal module, electrodes for applying charges to the liquid crystal material, and an output polarizer (also called an analyzer). For example, LCD panel 20 can be a conventional thin-film transistor (TFT) active matrix LCD panel. Conventional liquid crystal panels use a twisted-nematic liquid crystal orientation. LCD panel 20 modulates the polarized light incident upon it to form an image to be displayed. The modulated light is directed to a projection lens 22 for magnifying and focusing the image onto a screen (not shown).
LCD panel 20 typically includes an input polarizer to further polarize the incident light to achieve a higher degree of polarization. The input polarizer is needed because PCS plate 16 is only capable of polarizing about 80% of the light. Also, some depolarizatoin can occur as light propagates through other optical elements, such as the condenser lens. Therefore, an input polarizer is placed directly before LCD panel 20 to removes light that is not polarized in the desired orientation. In this manner, the input polarizer ensures that the light transmitted through LCD panel 20 is completely and precisely polarized.
While the conventional LCD projectors such as LCD projector 10 is useful in data projection applications, the conventional LCD projector architecture has several disadvantages. First, the lens array used to collect the light from the light source has a narrow acceptance angle, typically less than xc2x13 degrees. The narrow acceptance angle of the lens array limits light collection, thus introducing inefficiency in the optical system.
Another disadvantage associated with the conventional LCD projector architecture is the color gradient problem that can occur with the use of the lens array/PCS system in a three-panel LCD projector. FIG. 1 illustrates a single-panel LCD projector. A three-panel LCD projector can be built using any conventional architecture. In one example, one lens array/PCS systems is used and two color separation plates are included in the optical path between the PCS system and the three LCD panels. The color separation plates typically include a color dichroic film for separating the white light from the light source into three separate color components. Most color separation plates are designed to work with light impinging upon it at a 45 degree angle. However, when the lens array system is used, the light transmitted through lenslets on one side (e.g. the left edge) of the lens array go through the color separation plate at different angles than light transmitted through lenslets on the center or right side of the lens array. As a result, the projected image may suffer from a color gradient problem. Attempts at eliminating these color gradients are not satisfactory because they usually involve using xe2x80x9ccleanupxe2x80x9d filters which cause the loss of final system brightness.
Finally, LCD projectors are typically larger in size than corresponding DLP projectors because they use lens array systems. This is because the lens array, the PCS system and the condenser lens are larger components than the light integrating devices typically used in DLP projectors. The smallest DLP projectors are typically 2-5 lbs. as compared to typical LCD projectors which are 5-8 lbs. LCD projectors are thus not well suited for portable applications. Therefore, it is desirable to provide an LCD projector that is more compact but with improved brightness to improve the portability of the equipment, while avoiding the aforementioned disadvantages.
FIG. 1 illustrates an LCD projector using a transmissive LCD panel. LCD projectors using a reflective LCD panel, such as liquid crystal over silicon (LCOS) panels, are all well known. Reflective LCOS projectors operate by shinning light onto an image forming reflective chip. The light reflected off the image forming reflective chip forms the image to be projected and displayed. In the case of an LCOS projector, the image forming device is a silicon chip with a liquid crystal panel built over the surface of the silicon chip. The liquid crystal cell on the LCOS chip modulates the incident light on a pixel by pixel basis to generate the desired image.
DLP projectors are also well known in the art. In a DLP projector, the image forming reflective chip is a Digital Micromirror Device (DMD) having constructed thereon approximately half to three-quarters of a million of microelectromechanical mirrors. The mirrors can tilt to reflect a predetermined amount of light to form the desired image. To generate color images, the light from the light source is beamed through a rotating color wheel. Separate red, green and blue fields of information are generated sequentially in time. The speed of rotation is fast enough (such as 60 times per second for each color) that the human eye perceives a full color image.
Conventional DLP projectors usually include some sort of light integration mechanism in the optical path to homogenize the light from the light source. This is usually done to enhance the uniformity of the light that illuminates the reflective chip. The light integration mechanism of choice typically is a light tunnel or a tunnel integrator. Because the light integration component is generally small, DLP projectors are smaller than LCD projectors. In addition, because the light coming from a tunnel integrator does not need to be polarized for the DLP chip to function, the smaller DLP projectors can be brighter than similarly constructed LCD projector.
Recently, a sequential color recapture (SCR) technique has been introduced in a DLP projector which uses a tunnel integrator with a mirrored input end and a color wheel with RGB (red-green-blue) dichroic coating at the output end. The SCR technique is described in an article entitled xe2x80x9cSequential Color Recapture and Dynamic Filtering: A Method of Scrolling Color,xe2x80x9d by D. Scott DeWald, Steven M. Penn, and Michael Davis, SID 01 Digest, p.1076-1079, 2001. The SCR technique improves light efficiency by recycling unwanted primary color light reflected from the color wheel in the tunnel integrator and recaptured by the mirror on the input end of the tunnel. In the paper, a gain of 80% over the conventional color sequential method is reported.
It is desirable to provide an LCD projector that can achieve the brightness level and image uniformity characteristics matching that of a DLP projector. It is also desirable to provide such an LCD projector which is compact and suitable for portable applications.
According to one aspect of the present invention, a projection display system includes a light source emitting a randomly polarized light and a polarization recapture system. The polarization recapture system includes a tunnel integrator and a transmitting/reflecting polarizer. The tunnel integrator has an input end and an output end where the input end includes an input aperture and a reflective inside surface coated with a quarter wave retarder. The transmitting/reflecting polarizer is coupled to the output end of the tunnel integrator. In operation, the randomly polarized light from the light source arrives at the input end of the tunnel integrator and enters the tunnel integrator through the input aperture. The polarization recapture system transmits light having a first polarization as polarized light output and recycles light having a second polarization orthogonal to the first polarization. In one embodiment, the transmitting/reflecting polarizer is a wire grid polarizer.
Specifically, the light having the second polarization is reflected by the transmitting/reflecting polarizer back into the tunnel integrator. The polarization recapture system recycles the light having the second polarization by reflecting the light from the reflective inside surface and reorienting the light having the second polarization to light of the first polarization using the quarter wave retarder.
In one embodiment, the projection display system is an LCD projector and the projection display system further includes a relay lens assembly, an LCD panel and a projection lens assembly whereby the polarized light output is modulated into light representative of an image to be displayed.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.